1. Field of the Invention
The present invention relates to a method of polishing a substrate, such as a wafer, and more particularly to a polishing method including the steps of measuring a film thickness of the substrate after polishing it and re-polishing the substrate if the film thickness has not reached a target value.
2. Description of the Related Art
Semiconductor devices are expected to become finer and finer in the future. In order to realize such a fine structure, a polishing apparatus, which is typified by a CMP apparatus, is required to have a more precise processing controllability and a high polishing performance. Specifically, a more accurate remaining film control (i.e., more accurate detection of a polishing end point) and improved polishing results (i.e., less defects and high planarity of polished surface) are required. In addition, a higher productivity (i.e., throughput) is also required.
In the present polishing apparatus, so-called “rework”, which is re-polishing of the wafer, may be performed in order to improve the polishing accuracy. This re-polishing includes the steps of transporting the wafer, which has been polished in the polishing apparatus, to an external film thickness measuring device, measuring a film thickness of the polished wafer by the film thickness measuring device, and polishing the wafer again in order to eliminate a difference between the measured film thickness and a target film thickness.
Flow of a conventional wafer polishing method will be described with reference to FIG. 1. The polishing apparatus is typically partitioned into a polishing section and a cleaning section. A wafer is firstly transported into the polishing section, where the wafer is placed in sliding contact with a polishing pad on a polishing table, while a polishing liquid (slurry) is supplied onto the polishing pad. The wafer is polished in the presence of the polishing liquid (step 1). The polished wafer is then transported to the cleaning section, where the wafer is cleaned (step 2) and further the cleaned wafer is dried (step 3).
The wafer that has been processed in this manner is then transported to a film thickness measuring device provided exterior of the polishing apparatus (step 4), and a film thickness of the polished wafer is measured by the film thickness measuring device (step 5). The film thickness of the wafer is compared with a predetermined target film thickness (step 6), and if the film thickness of the wafer has not reached the target film thickness, then the wafer is transported into the polishing apparatus again, where the wafer is re-polished, cleaned, and dried. Such re-polishing (which is so-called rework) is effective at realizing an accurate film thickness, but on the other hand it takes a certain time from the first polishing step to the re-polishing step, lowering the productivity (throughput).
According the above described polishing method, it is possible to adjust polishing conditions (such as a polishing time and a polishing pressure) for subsequent wafers based on the film thickness measurement result in the external film thickness measuring device. However, when the adjusted polishing conditions are applied to the wafer polishing process, several wafers have already been polished. This means that the adjusted polishing conditions are not reflected in polishing of those wafers. In order to reflect the adjusted polishing conditions in polishing of the next wafer, it is necessary to keep the next wafer waiting until the film thickness measurement of a preceding wafer is terminated and the adjustment of the polishing conditions is completed. However, such an operation results in a lowered productivity (throughput).